Walking away from the summit was POTUS' only play. Signing a comprehensive deal with KIM was what the MSM was expecting, I think. They, no doubt, had a "Trump is a Putin puppet" enhancement narrative lined up (which they still tried to push anyway.) Now, what can they say? They had already astroturfed up the hand wringing on how Kim couldn't be trusted and Trump was dangerous because he foolishly "loves" Kim. Now, Trump turns around and walks–making him look strong and independent. The MSM can't now go back on its narrative and say, oh, Kim's a good guy and Trump should have given him more of a chance (though, here again, they are pushing the edges of that as much as they can as well.)
This way, the pressure is off. Still guaranteed no more testing etc. But now, Pompeo and Co can work behind the scenes without the pressure of the spotlight and never ending hand wringing about how Kim can't be trusted and we're all being bamboozled. There's more that needs to be done, obviously–probably mostly to do with Iran and whatever common link their nuke programs had under the DS. Feels like there's more evidence to be recoverd in NK that's best done under the radar.